AB STRA CT The Croatian offshore hydrocarbon province is highly under-explored. From a total number of 128 exploration wells, 96 were drilled in the Northern Adriatic Sea, 27 in the Central Adriatic Sea and only 5 in the Southern Adriatic Sea. Most of exploration wells were drilled to test the Pliocene shallow sandstone biogenic gas play, which has been in production since the end of the twentieth century. However, the deep Mesozoic oil plays are still heavily under-ex- plored. By interpreting regional studies, offshore well datasets and 2D seismic lines acquired by Spectrum in 2013, this study identified two new interesting explorative plays: Structural Karst play (Triassic/Jurassic); Slope Calcitur- bidites play (Cretaceous/Eocene). 2D petroleum system modeling was performed along a representative interpreted seismic section to define the potential and the exploration risk of the two plays. The results show that all the critical elements for a working petroleum system are present for both plays. Several fluid-saturated traps are present in the Cretaceous and Jurassic reservoirs with an indicative total depth between 3500 and 5500 m and in water depth from 500 to 1200 m. For the Structural Karst play the main uncertainties are related to the quality and efficiency of the reservoir, while for the Slope Calciturbidites play the main uncertainty is related to the sealing efficiency. Keywords: Croatian offshore, deep target southern Adriatic Sea, petroleum system modeling, Mesozoic Carbonatic traps  Exploring the potential of deep targets in the South Adriatic Sea: insight from 2D basin modeling of the Croatian offshore  Nicoletta Mazzuca1, Alessandro Bruni2 and Thorsten Joppen3 1 Enel s.p.a, viale Regina Margherita 137, 00198, Rome, Italy; (nicoletta.mazzuca@enel.com) 2 Upstream Gas Division, Enel Trade s.p.a, via Arno 42, 00198, Rome, Italy 3 Schlumberger Information Solutions, Ritterstr. 23, 52072 Aachen, Germany doi: 104154/gc.2015.18 quisition of a good quality seismic dataset. On the Croatian side, high quality seismic data was only obtained in 2013, when Spectrum acquired 14,700 km of long streamer 2D seismic lines (yellow lines in Fig. 1). This seismic dataset provided the opportunity to reconstruct a geological cross- section along the south Croatian offshore and to interpret potential Triassic and Jurassic source rock systems as well as multiple carbonate platforms where it is possible to dis- tinguish changes in seismic facies and thickness from the Triassic to the Cretaceous (Fig. 2). The aim of this study is to evaluate the efficiency of the thermogenic petroleum systems for the two newly identified and under-explored plays in the southern Croatian offshore (Fig. 2). The Structural Karst Play (Triassic/Jurassic), formed by antiform traps in the carbonate karst of the Adriatic Carbon- 1. INTRODUCTION Hydrocarbon exploration in the Croatian offshore started with the Jadran-1 well, drilled in 1970. The first success came three years later when the Ivana gas field was discov- ered by the Jadran-6 well. This initial discovery was in the Pliocene shallow clastic biogenic gas play in the northern Adriatic Sea and was followed by the discovery of additional gas fields that are still in production. The hunt for oil in the central and southern Adriatic-Ionian basin, began later. The focus was the Eocene and late Cretaceous carbonate plat- form potential oil plays in the shelf area (BEJDIC, 2012), but a few wells encountered oil shows (e.g. Melita-1, Vlasta-1). As a consequence this area remained under-ex- plored. In contrast, the Rovesti and Giove wells in the Ital- ian Apulian platform margin led to interesting oil discoveries (Fig. 1). This exploration success was followed by the ac- Geologia CroaticaGeologia Croatica Geologia Croatica 68/3 237–246 12 Figs. Zagreb 2015 Geologia Croatica 68/3Geologia Croatica 238 is characterized by deposition of bituminous limestone in- terbedded with dolomite formations in syn-rift grabens. In the Middle Liassic, the main Tethyan rifting phase caused the break-up of the previously continuous carbonate platform (VLAHOVIC et al., 2005). As a consequence, two main pe- lagic domains were generated, the Umbria-Marche and South Adriatic Basins with limestone to siliciclastic sequences de- posited from the Middle Jurassic until the Paleogene (CATI et al., 1987). Two different carbonate platforms remained sep- arated: the Dalmatian-Adriatic to the east-northeast and the Apulian to the southwest. Southwest verging Dinaric com- pression and thrust ing, similar to that affecting the Ionian ba- sin to the south, affected the Dalma tian-Adriatic platform, developing a foredeep in the Late Eocene and Oligo-Mio- cene times. In a similar way, the east-vergent Apulian thrust- ing formed a foredeep during the Plio-Pleistocene (WRIG- LEY et al., 2014). Successive deltaic sequences commenced in the Paleocene until the present day, prograding from the Po valley of Italy and the Neretva River of Croatia, with lit- tle localized sediment supply from demolition of the Dinaric thrust belt. The only exception is the evaporitic deposition during the Messinian. The effect of the Messinian salinity crisis was the generation of a clear and easily recognizable unconformity separating the Oligo-Miocene from the Plio- Pleistocene sequence. This geological evolution favoured the development of a platform carbonate succession alternating with basin de- posits, constituting the elements for efficient deep petroleum systems (Fig. 3). Bituminous limestone rocks deposited in the Upper Triassic and Early Jurassic syn-rift grabens can have high source rock potential as indicated and supported by Italian analogs. ate Platform (AdCP) (VLAHOVIC et al., 2005), was created as paleo-highs from the Triassic to the Lower Jurassic and moved during the Tertiary due to transpression. The traps could be charged by hydrocarbons from Upper Triassic source rocks and sealed by deposits from the Lower Jurassic and Middle Cretaceous (Fucoidi Fm.); the Slope Calcitur- bidites Play consists of stratigraphic traps formed as a series of carbonate fans extended basinward and related to partial disintegration of the Upper Mesozoic AdCP during the end of the Cretaceous-Eocene period (VLAHOVIC et al., 2005). The traps could be charged by Upper Triassic and Lower Jurassic source rocks and sealed by Tertiary deposits. 2. GEOLOGICAL SETTING OF THE CROATIAN ADRIATIC OFFSHORE The regional geology and tectonic history of the Croatian Adriatic basin is well described in the literature (CASERO et al., 2012; GRANDIC et al., 2013; SHINER et al., 2013). The Adriatic basin developed on the ‘Adria microplate’, on the stable western margin of Gondwana’s Neo-Tethys, which began to rift and subside in the Permo-Triassic. The Perm- ian to Anisian sequences show siliciclastic beds or lime- stones and dolomites, interbedded by salt and gypsum de- posits. The Permian is correlated to wells and outcrops of onshore Croatia (GRANDIC et al., 2004). In the Carnian- Norian period, the main lithofacies in the Periadriatic region were the Dolomia Principale/Burano formations from the extensive tidal zone to the evaporitic platform. The Burano evaporites (mainly salt and anhydrite) are present in the cen- tral and southern sectors of the Adriatic Sea (MATTAVELLI et al., 1991). The end of the Triassic and the Lower Jurassic Figure 1: Location map and available data set. Mazzuca et al.: Exploring the potential of deep targets in the South Adriatic Sea: insight from 2D basin modeling of the Croatian offshore Geologia Croatica 239 The stratigraphic column representing the study area and its petroleum system elements was recon- structed from the regional geological setting, the inter- pretation of Croatian wells implemented with Italian well information and the interpretation of new seismic data acquired by Spectrum (Fig. 3). The identification of Triassic evaporites has a big impact on the Mesozoic petroleum system. Within the Adriatic margin the evaporitic deposits are pre- dominantly anhydritic to the west and a mix of anhy- drite and halite to the east (GRANDIC et al., 1997). These evaporitic sequences often represent the de- tachment surface of major thrusts, emphasizing the “structural ejection” (development of high structure) of both compressive and strike-slip tectonics. The Apulian platform margin was remarkably stable re- maining almost stationary from the Triassic to the Late Cretaceous (CASERO & BIGI, 2013). This sta- bility is not proven for the Croatian carbonate plat- form edge, where the eastern platform was deposited over mobile halite (CASERO & BIGI, 2013). Salt bounded car bonate rafts and complex margins condi- tions can justify porosity enhancement via karstifica- tion, remobilization of carbonate and reefal build ups (WRIGLEY et al., 2014). 3. MODEL METHODOLOGY Basin and petroleum systems modeling is an inte- grated multidisciplinary approach to track the evolu- tion of a basin through time as it fills with fluids and sediments that may eventually generate or contain hydrocarbons. The modeling process consists of two main stages: model building and forward modeling. Model building involves building a structural model and identifying the chronology of deposition and physical properties of each layer. Forward modeling performs calculations on the model to simulate sedi- ment burial, pressure and temperature changes, kero- gen maturation and hydrocarbon expulsion, migra- tion and accumulation. The calibration phase com pares simulation results with independent mea- surements (e.g. well data) to enable refinement of the model (AL-HAJERI et al, 2009). The output model helps understanding of expulsion timing and poten- tial migration paths existing in the system as well as the exploration risks. These processes may be exam- ined at several levels, and complexity typically in- creases with spatial dimensionality. Considering the restricted available dataset in the area of interest (Fig. 1), 2D modeling along the key cross section was per- formed using, the PetroMod 2D (2014) software. Pet- romod 2D solves deterministic computations based on the discrete numerical representation of layers subdi- vided into grid cells with uniform properties. The soft- ware simulate physical processes that act on each cell, starting with initial conditions and progressing by se- lected time increments to the present day. Fi gu re 2 : 2 D re pr es en ta tiv e in te rp re te d se is m ic se ct io n co ns id er ed in th e pr es en t s tu dy . Geologia Croatica 68/3Geologia Croatica 240 3.1. Model Building The depth-converted horizons coming from the key inter- preted seismic line were used as input for building the 2D model. The seismic line is located in the South Adriatic Sea and extends from the Croatian coastline to the Italy-Croatia offshore border (Fig. 4). The long streamer used during the new 2D seismic survey, revealed interesting deep reflectors that probably belong to Triassic formations. These reflectors are the first acoustic impedance contrasts below the Meso- zoic platform carbonates and may indeed represent low-ve- locity bituminous limestone deposits (Fig. 4). Oligo-Mio- cene and Plio-Pleistocene stratigraphic plays onlapping deeper structures are also interpreted. The Messinian unit is a characteristically strong reflector and may represent a clas- tic play south of the Gargano Arch. The interpretation resulted from examination of regional geological studies, 2D seismic sections and well data on the Italian and Croatian side. The main reflectors coming from the seismic interpretation are: the Sea Bed, Top of the Mi- ocene, Top of the Oligocene, Top of the Eocene, Top of the Fucoidi Marls (L. Cretaceous), Top of the Lias (Jurassic) and an horizon within the Triassic. Based on the wells stratigra- phy and regional thickness maps, several other units have been interpreted: Near Top Cretaceous, Top Lias Source Rock, near Top Mid-Lias (Massiccio equivalent) and Near Top Triassic (Fig. 4). For the deepest reflectors, no well cal- ibration was available. As the interpreted time horizons and the depth conversions were done on an incomplete dataset, the depth model is not accurate. For modeling purposes, 4 main horizons were added to the model on the basis of the Italian wells (Fig. 5): the base Fucoidi Fm. (30 m below the interpreted top); the base Liassic source rock (50 m below the interpreted top); the base Trias source rock (50 m below the interpreted top); the Basement (about 1500 m below the base of the Trias source rock). The facies model with the associated physical properties for the main petroleum system elements (sources, reservoirs and seals) was built on the basis of the evolution of the sedi- mentary sequence of the Dinarides, from the Paleozoic-Tri- assic clastic-evaporite sediments through the Mesozoic car- bonate-anhydrite sequence to the Tertiary clastics (Fig. 6). The main reservoirs were characterized as: • Triassic (Rhaetian), Dolomite (organic lean, sandy); • Jurassic (Lias), Dolomite (organic rich 50% and lean 50%); Figure 3: Reconstructed Strati- graphic Column with the main pe- troleum system elements. Mazzuca et al.: Exploring the potential of deep targets in the South Adriatic Sea: insight from 2D basin modeling of the Croatian offshore Geologia Croatica 241 • Jurassic (Malm), Dolomite 50% and Ooid 50%; • Cretaceous, Limestone (ooid grainstone); • Eocene, Limestone (ooid grainstone). The seals were considered to be the Tertiary deposits (Marl 50% and Conglomerates 50%), the Upper Cretaceous Fucoidi (Marl) and the source rock intervals. The Upper Triassic source rock was the bituminous Li- me stone inside Burano Formation (ANDRÈ & DOULCET, 1991). The second source was more speculative and mainly based on Liassic Italian analogous to the Emma Limestone formation, documented for the Grifone-1 well (MATTA- VELLI et al., 1993; CASERO, 2004). The geochemical parameters (Total Organic Carbon- TOC, Kinetics and Hydrogen Index–HI) used for the source rocks were (PEPPER&CORVI,1995): • Trias Source: TOC 1,7%, HI 700, Kinetics Type II S; • Lias Source: TOC 2%, HI 700, Kinetics Type II S. Bottom Hole Temperature (BHT) data from the logs of about 10 wells were collected, corrected for equilibrium tem- perature and interpreted to gain understanding of the trend of the geothermal gradient. The Horner method was used. It plots the measured temperature (at a given depth) from each of several logging runs, against log(T/(t+T)). The parameter t represents the length of time that the borehole was subjected to the cooling effects of the fluid, and T represents the time after circulation that the borehole has had to partially reheat. The best fit straight line results from the plot is extrapolated to cut the temperature axis where logT/(t+T) equals zero, reflecting the true formation temperature at that particular depth. We have assumed mean surface temperature to be 7˚C. Figure 4: Interpreted horizons used as input for the 2D basin modeling. Figure 5: Seismic horizons added for 2D basin modeling purpose. Geologia Croatica 68/3Geologia Croatica 242 The geothermal gradient of the study area varies from 10°C/km to 30°C/km and averages about 17°C/Km (Fig. 7). 3.2. Forward Modeling To run the simulation, three important boundary conditions were defined: 1. Pa- leo Water Depth (PWD); 2. Sediment Wa- ter Interface Temperature (SWIT); 3. Paleo Heat Flow (HF). Two main trends were used as input to simulate the paleo-envi- ronment that defines the PWD and SWIT: one to model the geometry of a stable shelf through geological time and the second to simulate the basin conditions. For the Pa- leo HF history only one trend was used to simulate the effect of the Triassic rifting event and the subsequent phase of thermal subsidence (cooling). Vitrinite data for only one well was useful for calibrating the reconstructed heat flow history. Two main erosional events, during the Messinian and the Late Oligocene and one hiatus event during the end of the Cretaceous were in- troduced in the model. This missing over- burden was reconstructed based on the well data and regional geological data (CATI et al., 1987, GRANDIC et al., 1997, WRIGLEY et al., 2014). 4. MODEL RESULTS The starting point of this analysis was the understanding of the hydrocarbon poten- tial and the associated exploration risks of deep Mesozoic plays. The results can be summarized as fol- lows: • The risk of overmature conditions with consequent source depletion seems to be absent considering the low reconstructed geothermal gradi- ent and related heat flow. Maturity model results confirm that both source rocks are within the oil win- dow and that they can efficiently generate and expel hydrocarbons (Fig. 8); • There is a significant divergence be- tween the quantity of hydrocarbons expelled and the quantity accumu- lated in the reservoirs, especially during the Lower Cretaceous (Fig. 9). The main reason seems to be the timely mismatch between expulsion and the deposition and compaction of a working seal. Figure 6: Facies Model with the definition of Petroleum System Elements across the studied seismic line. Mazzuca et al.: Exploring the potential of deep targets in the South Adriatic Sea: insight from 2D basin modeling of the Croatian offshore Geologia Croatica 243 After calibrating and simulating the 2D model, the chart with all the features of the petroleum system was completed (Fig. 10). The biggest risk for the efficiency of the studied petroleum systems is the late completion of the traps for Cre- taceous/Eocene reservoirs (dark blue row). The seal rock was deposited in the Oligocene and compaction is ongoing through the Miocene, when most of the hydrocarbons were expelled. A more efficient compaction can be expected in the central part of the basin, where a thicker sequence of Oli- gocene and Miocene sediments can be observed. The Lower Jurassic reservoir could have the best work- ing petroleum system assuring hydrocarbon accumulations Figure 7: Geothermal gradient in the South Adriatic Croatian Offshore. Figure 8: Maturity stage at the present day: The Lower Jurassic source is in the main oil window and the Triassic source rock is mainly in the oil window and partially in the wet gas window. Geologia Croatica 68/3Geologia Croatica 244 Figure 9: Graph showing the total hydrocarbons expelled (orange), accumulated (red) and lost (green) in the system. Figure 10: Petroleum Systems Elements chart derived from a pseudo-well in the central part of the 2D basin. (Fig.10). In fact, the Triassic source rock starts to expel when the reservoir is deposited and the Lias source works as a seal. A pseudo-well was introduced in the central part of the basin. In this area the reconstructed burial history shows that the source rocks are buried at their deepest and up-dip fluid migration, towards the highs of both structural traps, was possible. The significant difference of generation between the two source rocks is related to the slow deposition of sed- iments in the basin between 180 and 30 Ma (Fig. 11). The Jurassic source barely reached 50% transformation of kero- gen, which explains the minor contribution to the accumula- tions observed in the model (Fig. 11). The final migration model shows that fluid saturated structures are present in both plays (Fig. 12). Also, consid- ering the actual uncertainties, the total depths of the Creta- ceous traps are around 3500 m and 5500 m for Jurassic traps, in water depth between 500 and 1200 m. This reservoir depth could have a negative impact on economic viability. 5. CONCLUSIONS All the critical elements for a working petroleum system are present in the Lower Jurassic structural carbonate karst play and in the Cretaceous slope calciturbidites play. The exis- tence of liquid hydrocarbons coming from Triassic and Ju- rassic source rocks into the studied petroleum systems at the present day seems to be possible (Fig. 8 and 12). For the Structural Karst play, which is highly under-ex- plored, there are still uncertainties concerning reservoir qual- ity (no porosity data) and efficiency (no well testing per- formed). For the Mid-Cretaceous/Eocene platform margin, there are significant uncertainties related to the possibility of find- ing significant accumulation in the traps. In fact, a temporal mismatch between expulsion and the deposition and com- paction of a working seal (Messinian, Top Oligocene and Top Cretaceous) could have facilitated hydrocarbon leakage to the surface. Mazzuca et al.: Exploring the potential of deep targets in the South Adriatic Sea: insight from 2D basin modeling of the Croatian offshore Geologia Croatica 245 Figure 11: Transformation of hydrocarbons (%) from kerogen through time at pseudo-well location for both source rocks. 6. DISCUSSION At the end of this study, the following points remain open for discussion in order to reduce risk in the area: • new ongoing exploration activities, since the last li- censing round in 2014, could reduce the uncertainties related to the missing hard-dataset. This could facili- tate well seismic calibration, core analyses and source rock geochemical characterization; • build a 3D petroleum systems model, when a new 3D seismic survey is acquired, to quantify the hydrocarbons in the system passing from play to prospect analysis; Figure 12: Potential new accumulations in Lower Jurassic and Mid Cretaceous traps. 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